This invention relates to memory cells and more particularly relates to content addressable memory cells.
Many memory devices store and retrieve data by addressing specific memory locations. As a result, this path often becomes the limiting factor for systems that rely on fast memory access. The time required to find an item stored in memory can be reduced considerably if the stored data item can be identified for access by the content of the data itself rather than by its address. Memory that is accessed in this way is called content-addressable memory (CAM). CAM provides a performance advantage over other memory search algorithms (such as binary and tree-based searches or look-aside tag buffers) by comparing the desired information against the stored data simultaneously, often resulting in an order-of-magnitude reduction of search time.
A CAM cell is the basic circuit determining the speed, size and power consumption of CAM systems. Known CAM cells employ a match circuit that changes state depending on whether there is a match or mismatch between data stored in the cell and test data. In the past, the match transistor has provided an unreliable indication of a match because, under some conditions, the voltage transmitted to the gate of the match transistor has been in a range of values between the VDD supply of the cell and the reference potential used in the cell, such as ground potential. This range of values provides an ambiguous indication that is difficult to interpret. In addition, the circuitry involved in determining whether there is a match condition typically has drawn current from the bit lines supplying the test data. This current draw increases the power consumption of the cell, because the current is conducted through the bit line. The bit line driver must be larger because all the cells connected to the same bit line draw current. In some other circuits, there are two series transistors to detect a match. This makes the matching response slow. This invention addresses these problems and provides a solution.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.
One apparatus embodiment of the invention is useful in a content addressable memory cell including a circuit operating from a predetermined supply voltage for storing a first bit of data at a first point and a second bit of data at a second point. The second bit of data is the complement of the first bit of data. The first bit of data and the second bit of data are compared with a third bit of test data transmitted on a first line and a fourth bit of test data transmitted on a second line. The fourth bit of data is the complement of the third bit of data. In such an environment, the comparison is aided by apparatus comprising in combination a first transistor comprising a first gate and also comprising a first circuit path switchable to a first state in response to a first predetermined relationship between the first and second bits and the third and fourth bits and switchable to a second state in response to a second predetermined relationship between the first and second bits and the third and fourth bits. A second transistor comprises a second gate coupled to the first line and comprises a second circuit path coupling the first point to the first gate. A third transistor comprises a third gate coupled to the second line and comprises a third circuit path coupling the second point to the first gate.
One method embodiment of the invention is useful in a content addressable memory cell including a circuit operating from a predetermined supply voltage and a predetermine reference potential for storing a first bit of data and a second bit of data. The second bit of data is the complement of the first bit of data. The first bit of data and the second bit of data are compared with a third bit of test data transmitted on a first line and a fourth bit of test data transmitted on a second line. The fourth bit of data is the complement of the third bit of data. In such an environment, the comparison is aided by employing a switching transistor comprising a gate in a method comprising in combination generating a first switching voltage substantially corresponding to the supply voltage in response to a first predetermined relationship between the first and second bits and the third and fourth bits while drawing substantially no current from the first line and second line. A second switching voltage substantially corresponding to the reference potential is generated in response to a second predetermined relationship between the first and second bits and the third and fourth bits while drawing substantially no current from the first line and second line. The first switching voltage is transmitted to the gate of the switch transistor in response to the first predetermined relationship, and the second switching voltage is transmitted to the gate of the switch transistor in response to the second predetermined relationship.
By using the foregoing techniques, the data in a CAM can be compared with test data with a degree of accuracy and reliability previous unattained. In addition, the power consumption of the cell is reduced.
These and other advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.